The present invention relates to liquefied natural gas (LNG) storage tanks built for marine vessels, and more particularly to the anchor and corner structures which are used to construct a heat insulating wall and sealing walls of an LNG storage tank built in a marine vessel for storage and transport of liquefied natural gas in a cryogenic state.
Liquefied natural gas (LNG) is formed through liquefaction of natural gas (i.e., a fossil fuel) and stored in LNG storage tanks. Depending on location, the LNG storage tanks are typically classified into on-land storage tanks built above or in the ground, and movable storage tanks installed on carriers such as vehicles, marine vessels, etc.
Since LNG is likely to explode if in a collision and is stored in a cryogenic state, the LNG storage tank must be firmly collision resistant and liquid-tight.
Compared with the on-land storage tank, which experiences little movement of the LNG, the LNG storage tank installed on vehicles or marine vessels experiences movement of the LNG, and is thus required to have a countermeasure capable of relieving mechanical stress caused by the movement of the LNG. In this regard, since the LNG storage tank installed in the marine vessel with the countermeasure against the mechanical stress can also be used as the on-land storage tank, the structure of the LNG storage tank installed in the marine vessel will hereinafter be described as an example.
FIG. 1 is a cross-sectional view schematically showing a marine vessel in which a conventional LNG storage tank is installed.
Referring to FIG. 1, the marine vessel 1 with the conventional LNG storage tank has a double walled hull, which comprises of an outer wall 16 and an inner wall 12 formed inside the outer wall 16. The inner and outer walls 12, 16 of the marine vessel 1 are connected to each other by connection ribs 13. In some cases, the marine vessel 1 can be made of a single walled hull.
The interior of the hull, that is, the interior of the inner wall 12, can be divided by one or more partitions 14. The partitions 14 can be formed by known cofferdams, which are installed in typical floating storage offloading vessels like the marine vessel 1 of FIG. 1.
Each of the inner spaces divided by the partitions 14 can be utilized as a storage tank 10 to store a cryogenic liquid such as LNG.
Here, an inner peripheral surface of the storage tank 10 is sealed liquid-tight by a sealing wall 50. In other words, the sealing wall 50 defines a single storage space with a plurality of metal plates welded together, so that the storage tank 10 can store and transport LNG without any leakage.
The sealing wall 50 in direct contact with LNG in the cryogenic state can be formed with corrugation to endure the temperature variation caused by the loading and unloading of LNG, as known in the art. The sealing wall 50 is connected to the inner wall 12 or the partition 14 of the vessel 1 by a plurality of anchor structures 30. Thus, the sealing wall 50 cannot be moved with respect to the hull.
A heat insulating wall is arranged between the sealing wall 50 and the inner wall 12 or the partition 14 to form a heat insulating layer therebetween. The heat insulating wall includes a corner structure 20 placed at a corner of the storage tank 10, an anchor structure 30 placed around an anchor member (not shown), and a planar structure 40 placed on a planar section of the storage tank 10. In this way, the overall heat insulating layer can be formed on the storage tank 10 by the corner structure 20, anchor structure 30, and planar structure 40.
Here, the anchor structure 30 comprises of a rod-shaped anchor member directly connected between the sealing wall and the hull to secure the sealing wall to the hull, and a heat-insulating material surrounding the anchor member.
The anchor structure 30 mainly serves to support the sealing wall 50, whereas the corner structure 20 and the planar structure 40 mainly serve to support the load the LNG exerts on the sealing wall 50. The corner structure 20 and the planar structure 40 are not directly connected to the anchor structure 30.
FIG. 2 is a cross-sectional view showing a part of a conventional LNG storage tank disclosed in Korean Patent No. 499710 issued to the applicant of this invention.
Referring to FIG. 2, the conventional LNG storage tank 10 includes a primary heat insulating wall 24, 34, 44 and a secondary heat insulating wall 22, 32, 42 sequentially stacked on the bottom of the hull, and a secondary sealing wall 23, 33, 43 between the primary heat insulating wall 24, 34, 44 and the secondary heat insulating wall 22, 32, 42 to seal the heat insulating walls. In addition, a primary sealing wall 50 is placed on the primary heat insulating wall 24, 34, 44.
The LNG storage tank 10 constructed as described above further includes a corner structure 20 placed at an inside corner, an anchor structure 30 spaced a predetermined distance from the bottom, and a planar structure 40 slidably interposed between corner structures 20 or anchor structures 30. The corner structure 20, the anchor structure 30, and the planar structure 40 are manufactured as unit modules that can be assembled onto the storage tank 10. Then, with these structures assembled onto the storage tank 10, the primary sealing wall 50 is finally placed on the assembled structures to provide a liquid-tight seal to the heat insulating walls, thereby defining a space to store the LNG.
As shown in FIG. 2, each of the corner structure 20, the anchor structure 30, and the planar structure 40 contains the primary heat insulating wall 24, 34, 44, the secondary heat insulating wall 22, 32, 42, and the secondary sealing wall 23, 33, 43, which will be commonly defined as heat insulating wall structures 20, 30, 40.
Meanwhile, in each unit module of the heat insulating wall structures 20, 30 and 40, the secondary sealing wall and each of the heat insulating walls are bonded together by adhesives. Typically, the secondary heat insulating wall 22, 32, 42 includes polyurethane foam, an insulating material, and a plate bonded to a lower surface of the polyurethane foam. The primary heat insulating wall 24, 34, 44 includes the polyurethane foam, and a plate bonded to the upper surface of the polyurethane foam by an adhesive. In addition, the primary sealing wall is positioned on the primary heat insulating wall 24, 34, 44, and welded to the anchor structure 30.
The secondary heat insulating wall 42 of the planar structure 40 is formed at a lower end with a flange 42a greater than the secondary heat insulating wall 42. The flange 42a is fitted into a groove formed in the lower end of the anchor structure 30 to slide somewhat therein.
In the construction shown in the drawing, the anchor structure 30 includes an anchor support rod 36, a securing member 37 positioned at a lower portion of the anchor structure 30, the primary anchor heat insulating wall 34, the secondary anchor heat insulating wall 32, and the secondary sealing wall 33 interposed between the primary anchor heat insulating wall 34 and the secondary anchor heat insulating wall 32. The anchor support rod 36 is connected at one end to the primary sealing wall 50, and at the other end to the inner wall 12 of the hull via the securing member 37.
The anchor support rod 36 of the anchor structure 30 has an upper end welded to the primary sealing wall 50.
Furthermore, the anchor structure 30 is positioned at a connection between the adjacent planar structures 40 to connect the planar structures 40, which are secured to the inner wall 12 or the partition 14 defines the storage tank 10. The securing member 37 of the anchor structure 30 is placed around the anchor support rod 36.
As such, since the heat insulating wall structures of the conventional LNG storage tank comprise the primary and secondary heat insulating walls, and the primary and secondary sealing walls, the conventional LNG storage tank is complicated in overall construction, in particular, in construction for connecting the secondary sealing wall, which makes it difficult to construct the heat insulting walls. Furthermore, the complexity in construction and installation of the anchor structure or the connecting structure for the secondary sealing walls deteriorates the reliability in the sealing properties of the secondary sealing wall, which can cause leakage of the LNG.
Furthermore, for the conventional anchor structure 30 which connects the inner surface of the hull and the primary sealing wall 50 via the anchor support rod 36, and the conventional corner structure 20 which supports only the load of the LNG exerted on the sealing wall 50 without supporting the sealing wall 50, it has been required to further improve the capability of absorbing stress occurred from the thermal deformation of the storage tank or deformation of the hull resulting from the loading and unloading of the LNG in the cryogenic state.
In order to achieve reduce the boiled off gas (BOG), which is a loss caused by vaporization of the LNG in the cryogenic state, and simplification in the construction and manufacturing process while solving the aforementioned problems, an LNG storage tank having a new construction completely different from that of the conventional LNG storage tank has been suggested. As a result, there is a needs for an improved anchor and corner structures corresponding to the new LNG storage tank.